1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine and a printer using electrophotographic recording technology and electrographic recording technology, and particularly to an image forming apparatus having a developing device changing rotary.
2. Related Background Art
As image forming apparatuses capable of outputting color images with use of toners of a plurality of colors, there are a developing device changing rotary type which has a rotary holding a plurality of developing devices and switching the developing devices opposing to a photosensitive member by rotating the rotary, a tandem type in which a plurality of photosensitive members are aligned, and the like.
The developing device changing rotary type has the advantage that the photosensitive member does not need to be provided for each color and the apparatus can be made compact, and the tandem type has the advantage of being capable of printing at a high speed because it can perform parallel processing of toner image formation onto each photosensitive member. Each type has its own advantages and disadvantages, and the types are selected in accordance with the concepts of products.
FIG. 3 is a perspective view of a changing rotary mounted on the developing device changing rotary type of image forming apparatus which becomes a reference for understanding the present invention. FIG. 3 shows the state, in which developing cartridges are removed from the rotary, and in an actual state of use, four developing cartridges are attached to the rotary.
Circular discs 107 and 110 are placed at both ends in a longitudinal direction of the rotary, and both discs are connected by a plurality of metal reinforcing stays 112 (four in this example). These four stays allow the rotary to have torsional rigidity. A gear 107a is provided at a side of the disc 107, and power of a motor 101 is transmitted to the gear 107a via gears 102 to 106, whereby the rotary is rotated.
When the rotary is rotated by transmitting the power to the gear 107a provided at only one side of the rotary like this, it is necessary to secure sufficient rigidity so that the rotary is not twisted. Accordingly, a number of reinforcing stays are necessary as described above, the number of components is increased, and the weight is also increased. In addition, it is necessary to bring both the discs 107 and 110 accurately in phase with each other at the stage of manufacture of the single body of the rotary, and therefore the manufacture cost is increased.